Using SM mode you can create a race on any body that is elligible for life. When you setup all their starting conditions (population, facilities number etc), the top left of the window has a check box to make this race an NPR or player race.
Just be careful if you are creating a bunch of empires all on that one body, each colony contributes to the overall maximum population for that body, if you go over that by creating a bunch of 4bn pop colonies, then in the first 5 day turn you each will suffer massive population loss and all the adde effects (facilities not working etc).